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Abstract

Hybrid nanomaterials
where active battery nanoparticles are synthesized
directly onto conductive additives such as graphene hold the promise
of improving the cyclability and energy density of conversion and
alloying type Li-ion battery electrodes. Here we investigate the evolution
of hybrid reduced graphene oxide–tin sulfide (rGO-SnS2) electrodes during battery cycling. These hybrid nanoparticles are
synthesized by a one-step solvothermal microwave reaction which allows
for simultaneous synthesis of the SnS2 nanocrystals and
reduction of GO. Despite the hybrid architecture of these electrodes,
electrochemical impedance spectroscopy shows that the impedance doubles
in about 25 cycles and subsequently gradually increases, which may
be caused by an irreversible surface passivation of rGO by sulfur
enriched conversion products. This surface passivation is further
confirmed by post-mortem Raman spectroscopy of the electrodes, which
no longer detects rGO peaks after 100 cycles. Moreover, galvanostatic
intermittent titration analysis during the 1st and 100th cycles shows
a drop in Li-ion diffusion coefficient of over an order of magnitude.
Despite reports of excellent cycling performance of hybrid nanomaterials,
our work indicates that in certain electrode systems, it is still
critical to further address passivation and charge transport issues
between the active phase and the conductive additive in order to retain
high energy density and cycling performance.

Introduction

Li-ion batteries (LIBs)
have become a ubiquitous energy storage
solution with applications ranging from consumer electronics to electric
vehicles. To fulfill the increasingly demanding requirements of these
applications, there is growing need for batteries with higher capacity,
rate capability, and greater cycling stability. Currently, graphite
is used as the anode in most commercial LIBs, yet because of its limited
theoretical capacity1 (~372 mAh g–1 based on LiC6 stoichiometry), alternative
anode materials with different chemistries and more complex nanostructures
are being developed. Metal sulfides (e.g., MoS2, SnS2, CuS, and WS2)2−8 are being investigated because of their high energy density and
because some of them (e.g., tin sulfide, SnS2) are easily
processable, low-cost, and environmentally benign.9

The layered dichalcogenide structure of SnS2 is composed
of tin atoms that are sandwiched between hexagonally close-packed
sulfur atoms. Similar to insertion electrodes, SnS2 can
intercalate Li ions between its layered structure, but eventually
it undergoes a conversion reaction followed by an alloying reaction
with Li+ ions.10,11 In general, pure SnS2 electrodes are cyclable only at low rates (e.g., 0.1C), and their capacity fades to more than half of their
initial capacity in just a few cycles,12,13 followed by
a further continuous capacity fade.11,14 At higher
rates (e.g., 1C and above), the pure SnS2 electrodes exhibit severe capacity fade because of irreversibility
in the above two reactions and large volume expansions. In addition,
these electrodes exhibit high polarization and low conductivity across
electrodes over battery cycling, and these issues severely limit the
application of pure SnS2 electrodes as anodes especially
with low carbon content (less than ~10 wt %). Table S1 in the Supporting Information summarizes the performance
of some of the pure SnS2 electrodes reported in the literature.

This capacity loss is often partially mitigated by using nontraditional
conductive additives such as graphene, reduced graphene oxide (rGO),14−16 carbon nanotubes,17 and graphitic carbon-coatings.18 The high surface area, flexibility, and mechanical
resilience of graphene, as well as its morphological compatibility
with two-dimensional nanostructures such as SnS2 platelets
have been shown to have beneficial effects on capacity and cycling
stability.9,14−16,19−26 Such advantages, in combination with nanostructuring of SnS2, offer short Li-ion diffusion paths and reduces crack formation
of the electrode material.4,12,27,28

A particularly interesting
approach to improve the interface between
both materials is to synthesize SnS2 nanoparticles directly
on the surface of conductive additives such as rGO or carbon nanotubes
(CNTs) to form “hybrids” rather than physical mixing
of the components to form “composites”.29 This can lead to enhanced charge transport across electrodes
due to the efficient charge transfer between the inorganic active
particles and the carbon conductive additive30 and has been found to result in improved capacity retention in LIBs11,15,21.

In this paper, we investigate
the evolution of rGO-SnS2 electrodes with particular attention
to the electrical accessibility
of active particles over battery cycling. The SnS2 nanoparticles
are synthesized directly on the rGO surface using a solvothermal microwave
reaction. This microwave process allows a simultaneous reduction of
GO to rGO. Here we provide novel insights related to the evolution
of rGO-SnS2 electrodes over many cycles using electrochemical
impedance spectroscopy (EIS), galvanostatic intermittent titration
(GITT), differential capacity analysis (dQ/dV), and post-mortem studies after extended cycling using
scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman
analyses. These measurements indicate also these hybrids can show
a strong passivation of the rGO surface, which is detrimental to battery
cycling performance.

Experimental Section

Preparation of rGO-SnS2 Hybrids

In a typical
synthesis, 15 mg of GO was synthesized using the Tour method31 and subsequently sonicated in 7.5 mL of dimethylformamide
(DMF) for 1 h. Na2S·9H2O and SnCl4·5H2O were mixed in the molar ratio 2:1 as adapted
from previous reports32 and added to the
GO dispersion. The mixture was loaded into an Anton Parr microwave
hydrothermal reactor and heated to 160 °C for 16 h while stirring.
The microwave power was automatically controlled by the reactor to
maintain the desired temperature. The reaction product was washed
in acetone and deionized water via vacuum filtration and dried on
a hot plate at 60 °C for 12 h.

The resulting rGO-SnS2 powder was then mixed with polyvinylidene fluoride (PVDF)
in a weight ratio of 93:7. The composite slurry was coated onto Cu
current collectors and dried at 120 °C for 24 h. CR2032 type
cells were assembled in an Ar-glovebox, and pure Li metal was used
as both reference and counter electrode. Glass fiber papers were used
to separate the electrodes, and 1 M LiPF6 in diethyl carbonate
and diethyl methyl carbonate in volume ratio 1:1 was used as the electrolyte.

Instrumentation

Characterization of the morphology
of rGO-SnS2 was performed by scanning electron microscopy,
energy dispersive X-ray analysis (EDX, Hitachi S5500), and transmission
electron microscopy (FEI Philips Tecnai). X-ray diffraction was measured
by a Bruker D8 Advance instrument. Raman spectra was obtained using
a Ezraman-N instrument. Electrochemical measurements were carried
out using a Biologic-VMP3 instrument. Thermogravimetric analysis was
carried out using a PerkinElmer TGA 4000 system.

Results and Discussion

In this work, we developed a microwave-enhanced solvothermal synthesis,
where hexagonally shaped SnS2 nanoplatelets were synthesized
directly on GO sheets as depicted in Figure ​Figure11. Important benefits of the microwave reaction
are both a fast synthesis and simultaneous reduction of the GO sheets,33 resulting in the formation of rGO-SnS2 hybrid nanostructures.

Schematic process flow of the SnS2 nanoplatelets synthesis
on rGO. First, GO is synthesized using the Tour method, followed by
a microwave solvothermal synthesis of SnS2 nanoparticles
on the GO sheets which are simultaneously reduced because of the microwave
irradiation....

Figure ​Figure22 shows the
morphology of our rGO-SnS2 hybrid nanostructures using
SEM and TEM. The SnS2 platelets are typically ~100
nm wide and are randomly distributed on rGO sheets of size ~5
μm (Figure ​Figure22a).
EDX analysis of a few SnS2 platelets indicates an atomic
ratio of 2:1 of sulfur and tin atoms, respectively (Figure ​Figure22b). Furthermore, the SEM analysis
suggests that the SnS2 nanoplatelets tend to nucleate on
the rGO sheets rather than in the bulk solution during the reaction,
thus ensuring SnS2 platelets are well-connected to rGO,
which should make the active phase (SnS2) electrically
more accessible during battery cycling. Additionally, the presence
of SnS2 particles prevents the restacking of rGO which
in turn acts as a mechanical support for the SnS2 particles.20 The lattice spacing of 0.6 nm measured by TEM
analysis (Figure ​Figure22c)
matches the (001) plane of layered hexagonal SnS2, an example
of which is shown in Figure ​Figure22d.

The XRD spectra in Figure ​Figure33a confirms the crystalline nature of the SnS2 platelets
(JCPDS 00-023-0677). The peak at ~15.1° corresponds to
the (001) plane and a lattice parameter for c of
~5.9 Å, suggesting a well-stacked layered structure along
the c axis. Based on the Scherrer equation, the thickness
of the SnS2 platelets was calculated to be around ~25
nm according to the (001) peak. The Raman spectra (Figure ​Figure33b) shows the characteristic
D and G peaks for graphene oxide at 1350 and 1590 cm–1, respectively, and a peak at 311 cm–1 which corresponds
to the A1g mode of SnS2.34 The intensity ratio of the D to G peaks in graphene oxide
is indicative of the degree of reduction. In this case, ID/IG was greater for rGO-SnS2 (1.08) compared to unreacted GO (0.92) as shown in the inset
of Figure ​Figure33b, confirming
reduction of GO to rGO during the synthesis due to microwave heating.33

(a) XRD results and (b) Raman spectra of rGO-SnS2 with
an inset for normalized intensity of rGO-SnS2 and GO peaks,
showing the increased D/G ratio of rGO compared to GO.

Next, we investigated the electrochemical performance
and phase
transformation of rGO-SnS2 electrodes using galvanostatic
charge–discharge, cyclic voltammetry, EIS, GITT, and dQ/dV analyses. The rGO-SnS2 cells
were cycled at room temperature between 3 and 0 V vs Li/Li+, and in some cases after a few initial cycles the charging potential
was limited to ~2.6 V or less (see additional discussion below).

In SnS2 electrodes, the first cycle capacity is typically
the sum of a large irreversible capacity (theoretical ~587
mAh g–1) originating from conversion reactions
and a reversible capacity (~645 mAh g–1) from alloying reactions.35,36Figure ​Figure44a shows the initial capacity of our electrodes
reaches ~1600 mAh/g at 0.1 A/g in the first cycle.9 The cells were cycled continuously with increasing
rates and delivered a capacity ~400 mAh/g at 1.2 A/g and sustained
a capacity of over ~600 mAh/g when the rate was reverted to
0.1 A/g. In the first cycle in Figure ​Figure44a, the plateau at ~1.4 V vs Li+/Li
corresponds to the conversion (SnS2 + 4Li+ +
4e– → Sn + 2Li2S), where Li ions
first intercalate between SnS2 layers and react with sulfur
to produce Li2S and Sn. The lower plateau at around 0.3
V vs Li+/Li corresponds to alloying reactions (Sn + xLi+ + xe– ↔ LixSn), where metallic Sn from
the above conversion reaction reacts with Li+ ions to form
LixSn alloy (0 ≤ x ≤ 4.4). Figure ​Figure44b shows the cycling of a rGO-SnS2 electrode at 0.1 A/g,
where a large drop in capacity can be noticed after only 25 cycles.
This capacity fade is even faster at higher rates, especially above
1 A/g (see Figure ​Figure44b).

Electrochemical characterization of rGO-SnS2 electrodes:
(a) galvanostatic cycling behavior at different current densities;
(b) capacity evolution during cycling at 0.1 A/g; (c) differential
capacity (dQ/dV) analysis of the
first cycle and after a round...

To investigate the rapid phase
transition of SnS2 and
the capacity fade associated with these processes, we performed differential
capacity analysis37 dQ/dV (Figure ​Figure44c). In the dQ/dV plot, the
peaks can identify the effects of degradation mechanisms and changes
in the battery chemistries as a result of repeated lithiation and
delithiation of rGO-SnS2 electrodes in charge–discharge
cycles. In the first discharge (cathodic sweep), the small peaks from
1.9 to 1.6 V are due to the stepwise insertion of Li+ into
SnS2 particles leading up to conversion reactions. At 1.4
V, the prominent peak is due to the conversion reaction that produces
Li2S and Sn metal. At potentials below 0.7 V, metallic
tin (extruded as a result of Li+ insertion) begins to react
with Li+ and gives rise to a shoulder at 0.3 V due to the
formation of LixSny alloys. This potential regime (from ~1.2 V and below)
also involves the decomposition of electrolyte (SEI) which partly
accounts for the capacity loss from the first cycle to the second.
After cycling (up to 1.2 A/g as in Figure ​Figure44a), the dQ/dV profiles of the electrode look different. The conversion reaction
peak at 1.3 V (Li2S, Sn) disappears, but the broad shoulder
at 0.3 V due to the alloy formation is still present. This reveals
that the conversion reaction is in fact limited and becomes almost
nonreversible in a few cycles, which corroborates why more than half
the capacity is lost in just a few cycles. On the other hand, the
alloying reaction (LixSny) is more reversible as the broad peak in the charge process
(anodic sweep) around ~0.5 V is due to the dealloying reaction,
which accounts for the reversible capacity in the rest of the battery
life. Similarly, in the cyclic voltammetry (CV) profiles (Figure ​Figure44d), the shoulders
at ~1.3, 0.3 V and 0.2, 0.5 V vs Li/Li+ are indicative
of the above two reactions, while the broad shoulder at ~1.3
V (associated with the conversion reaction) eventually disappears,
in line with the galvanostatic charge–discharge cycles and
dQ/dV profiles. Figure ​Figure44e shows a GITT38 analysis of the electrodes in order to evaluate the Li
content as a function of charge–discharge potentials (V), which
suggests that in the first cycle, ~11 mol equivalent of Li
is inserted per formula unit of rGO-SnS2. This is in slight
excess of the actual number of moles of Li+ ions that should
be involved according to both of the above reactions, which can be
due to the presence of rGO sheets inserting additional Li+ ions. The proportion of rGO in the hybrid material is estimated
to be ~5% based on the ratios between the reagents in the synthesis
and thermogravimetric analysis (TGA) results (Figure S1).

The calculated chemical diffusion coefficient
of Li+ (D)39 from GITT measurements
shows a large variation as a function of the charge–discharge
potentials (V) of the cells in the first cycle (see the Supporting Information for more details on the D calculation). As shown in Figure ​Figure44f, D decreases from ~10–9 cm2 s–1 to 10–14 cm2 s–1 at potentials
where Li+ intercalates into SnS2 layers leading
to conversion reactions (at ~1.4 V) and alloying reactions
(at 0.3 V). First, the drop in D values at ~1.3
V is due to the conversion reaction of SnS2. Because this
basically involves a redox process, the solid-state Li diffusion in
Li-rich converted regions is severely disrupted. Second, as the Li
insertion continues in SnS2, the occupation of Li vacancies
is quickly diminished, which hinders Li ion mobility further. This
however seems to recover to some extent in the rest of the charge–discharge
states after the phase conversion (i.e., SnS2 into Li2S and Sn). At around ~0.3 V, the LixSn alloying reaction takes place, and this phase transformation
is reflected again in the drop of D values corresponding
to Li ion diffusion in LixSn phases (0
≤ x ≤ 4.4), in agreement with dQ/dV analysis (Figure ​Figure44c) and CV data (Figure ​Figure44d). These fluctuations in D values reveal a slow Li diffusion during the phase transformation
of SnS2 as these electrodes turn into amorphous or poorly
crystalline Li rich and alloy phases forming a gel-like matrix already
in the first discharge. During charging, D values
also fluctuate and drop considerably after ~1.5 V where the
decomposition of lithiated phases and oxidation of metallic Sn should
be taking place, most likely due to the accumulation of Li polysulfides.
This seems to limit the charging potential of these electrodes to
~2.6 V or less after the initial cycles. We found that the
Li diffusion in the cycled electrodes further decreases by a factor
of 10 or more (from D ~ 10–9 cm2/s to D ~ 10–10–10–11 cm2/s) after 100 cycles,
as shown in Figure ​Figure55a. As expected, at this stage of cycling there is not much variation
in D as a function of the charge–discharge
potentials because the electrode has become amorphous; this is confirmed
by XRD analysis(see the following discussion).

(a) Li ion diffusion
coefficient from GITT analysis on electrodes
cycled to 100 times; (b) EIS data on rGO-SnS2 electrodes
cycled a different number of times.

Despite the hybrid nature of the rGO-SnS2 electrodes,
the above-mentioned measurements indicate a clear loss in performance
after less than 100 cycles. The capacity drop can be due to many factors,
including pulverisation, the dissolution of polysulfides in the battery
electrolyte which can degrade the electrodes, and the formation of
insulating phases on conductive additives which can decrease electron
conductivity across electrodes.40 To better
understand the capacity fade over cycling, we performed EIS on these
electrodes, revealing that the electrode’s initial resistance
(~25 Ω) doubles in about ~25 cycles (~50
Ω) and increases up to ~4 times (~90 Ω)
after 50 cycles (Figure ​Figure55b). This may be caused by a passivation layer forming between the
active particles and the rGO additive by sulfur enriched conversion
products.41

To further understand
the evolution of the hybrid rGO-SnS2 nanoparticles in electrodes
during extended charge–discharge
cycles (although the capacity had faded significantly), batteries
were run for 50, 200, and 500 cycles at ~1 A/g and were subsequently
opened and analyzed in delithiated state by SEM, Raman, and XRD analyses.
Although the presence of PVDF binder in the electrode material and
the formation of a gel-like matrix after such vigorous cycling makes
high-resolution imaging by SEM difficult, the formation of large agglomerates
in the battery electrode is still observable in post-mortem SEM measurements
(see Figure S2). Figure ​Figure66a shows the XRD patterns indicating that
the electrode composition is mostly amorphous in all cases and that
no SnS2 was detected in completely delithiated electrodes,
as indicated by GITT and dQ/dV analyses.
Many of the XRD peaks can be assigned to metallic Sn (JCPDS 04-0673)
according to the alloying–dealloying reaction (LixSn); this is also evident in the CV and dQ/dV where the peak at ~0.5 V on
charge indicates LixSn dealloying which
releases metallic Sn. Similarly, Raman spectra show no peaks associated
with SnS2 phases. Interestingly, the presence of rGO was
only just detectable at 50 cycles and was no longer detectable after
200 cycles (Figure ​Figure66b). Faint signals for the presence of rGO were still detectable in
some cases when the battery was cycled at a slow rate (100 cycles
at 0.1 A/g; see the Supporting Information). Similar observation where the prominent D and G peaks of CNTs
were significantly reduced upon increasing the amount of sulfur onto
the CNT surface in Li–S battery electrodes was recently report
by Chen et al.41 The above measurements
suggest that from the early cycles, converted products such as polysulfide
phases might adsorb onto the rGO and compromise the interface between
SnS2 and rGO. Eventually, as the batteries are cycled further,
the progressive accumulation and the increased binding of polysulfides
might completely passivate the rGO sheets irreversibly, which then
compromise the battery lifetime.

Conclusion

This paper presents a
solvothermal microwave reaction where SnS2 nanoparticles
are synthesized directly on rGO conductive
supports. In the first cycle, GITT analysis confirms that the phase
conversion of SnS2 taking place via conversion and alloying
reactions is characterized by slow Li-ion diffusion. After the initial
cycles, the Li and Sn alloying–dealloying becomes the dominant
electrode reaction accounting for the sustained capacity as shown
by dQ/dV analysis. However, despite
the hybrid nature of our electrodes, we observe a fast decrease in
capacity after only ~25 cycles. EIS indicates that at this
stage, the impedance of the electrode has already doubled. GITT analysis
shows that Li-ion diffusion coefficient drops by an order of magnitude
between the 1st and 100th cycle. Post-mortem analysis of electrodes
was carried out after 50, 100, 200, and 500 cycles, and the results
further corroborate that rGO becomes irreversibly passivated. Our
findings therefore indicate that in some cases, and especially in
electrodes with low weight percent of carbon additives, hybrid nanomaterials
still require additional measures to retain effective charge transport
between the conductive additive and the active phase during battery
cycling.

Acknowledgments

M.H.M acknowledges the support
from EPSRC Cambridge NanoDTC,
EP/G037221/1. C.G and M.D.V acknowledge the support from ERC starting
grant HIENA-337739.